In a liquid crystal display device for a word processor, a personal computer, a TV, a liquid crystal display instrument or the like, in order to increase visibility, in a backlight unit, a light diffusing film or sheet (generally called a light diffusing plate; in the present specification, hereinafter this will be typically referred to as a light diffusing film) is used. The most primary optical characteristics required for the light diffusing film are total light transmittance of at least 90% for visible light and a haze of at least 85% thereto. The light diffusibility of the light diffusing film can be given, for instance, by giving concavities and convexities on a film surface, or by mixing and dispersing several tens % by weight of a light diffusing agent such as fine particles inside of the film or in a resin (binder) coated on a film surface.
As one in which the light diffusibility can be given by forming the concavities and the convexities on the film surface, light diffusing films in which the concavities and the convexities are formed on a surface of a transparent resin film made of polyethylene terephthalate resin, polymethylmethacrylate resin, or polycarbonate resin can be used. However, only by giving the concavities and the convexities on the film surface by embossing or sand blasting, it is difficult to obtain excellent light transparency and light diffusibility in combination.
As one in which the light diffusing agent such as fine particles is dispersed inside of a film, a light diffusing film in which in a transparent film made of polymethylmethacrylate resin, polycarbonate resin or the like, a light diffusing agent such as calcium carbonate, titanium oxide, glass beads, silica particles, polystyrene particles, silicone resin particles, cross-linked polymer particles or the like is dispersed can be used. These are disclosed in, for instance, JP-A-50-146646, JP-A-56-33677, JP-A-60-175303, JP-A-61-4762, JP-A-63-291002, JP-A-1-172801, JP-A-2-173701, JP-A-3-78701 and so on.
As one in which a light diffusing agent is dispersed in a solution in which a plastic or curable resin is dissolved in a solvent and coated on a film surface, a light diffusing film in which fine particles such as calcium carbonate, silica particles, acrylic polymer particles, silicone resin particles, polystyrene particles, urea resin particles, polyethylene particles, polycarbonate particles, polyvinyl chloride particles, cured melamine resin particles or the like can be used. These are disclosed in JP-A-1-172801, JP-A-6-138308, JP-A-7-209502, JP-A-7-218705, JP-A-9-113708, JP-A-11-160505 and so on.
In JP-A-160505, a method in which a liquid resin composition in which plastic particles are dispersed as the light diffusing agent in a UV-curable resin is coated on a transparent film followed by curing by irradiating with a UV light, and thereby a coating type light diffusing film is produced is also proposed.